1. Field of the Invention:
The present invention relates to a process for making iron powder or iron-based powder, without oxide impurities therein, which powders are suitable for making molded parts having a precise and controllable carbon content.
2. Description of the Background:
Most iron powders contain a defined carbon content that influences the properties of sintered parts made therefrom. These powders also contain impurities such as oxygen or nitrogen up to a level depending upon the process used to produce the powder. For example, carbonyl iron powders typically contain up to 0.315 wt. % of oxygen and 0.7 wt. % of nitrogen. During sintering, oxygen impurities in the powder can react with carbon and result in decarburization. Thus, an important concern of powder producers is to lower, as far as possible, the level of oxygen impurities in the powders.
A conventional process for producing iron powder is disclosed in F. L. Ebenhoech, Carbonyl Iron Powder Production, Properties and Applications, Progress of Powder Metallurgy, vol. 42, Princeton Ed., 1986. According to this process, treatment under pure hydrogen up to temperatures around 400.degree. C. facilitates the removal of oxygen impurities. However, it is difficult in practice to remove oxygen with a hydrogen treatment. It is known from D. R. Ryan and L. J. Cuddy, "Effect of Atmosphere Composition on the Sintering Behavior of Iron Powder Compacts, 1990 Advances in Powder Metallurgy," vol. 2, pp. 261-77, Princeton Ed., 1991, that FeO oxides can only be reduced under pure hydrogen at temperatures above 1100.degree. C. However, treatment of iron powder at that temperature is not possible since the powder particles would bind together to form a compact solid which is difficult to grind. Further, the partial reduction of oxides under a hydrogen atmosphere occurs with a simultaneous decarburization, in particular, since in the temperature range of 200.degree. C. to 400.degree. C., carbon reacts with hydrogen to produce methane.
In fact, very little information regarding the removal of oxygen impurities from iron powders without decarburization is available in the literature. However, a need exists for a process for making carbon-containing substantially oxide-free iron-based powders, which does not entail the removal of oxygen impurities by decarburization.